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You need to have a currency conversion table available.
This typically will contain the From currency, to currency, effective date and
the rate.
Search the internet as these are available but most companies would subscribe to
some service to get this information.

Changing the formatting will have no affect on the value.
IF the amount is 200 and it is in British pounds, converting it to USD will give
you a different value, say 150.
Changing the formatt to show a different currency will just show 200 with a
different currency.
You still need to convert the value based on a date.

As per Randy then we have look up table with the currency conversion. As per the date it will look up the table and do the currency conversion. That what we are doing it but we are using JDEDWARD or peoplesoft . to do it.

In order to convert from one currency to another, you need to get the currency
conversion rates.
Many places publish this on the web. Many companies subscribe to get this info
from bloomberg, etc.
It becomes a simple table with From currency, to currency, date and the rate.
you just do a lookup based on the currencies you are interested in and the date
and multply the amount by the rate for that day.

Randy,
I am in the same boat, I already have a conversion table and also my fact table has 5 different fields for sales for five different currencies. Now on my screens, I would like to provide a currency drop down and display the corresponding sales for the currency picked, any ideas of how to implement this in efficient way?